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Building Design that Pushes Engineering Limits

Architects designed a huge cantilevered canopy for the new Francis A Gregory library in Washington DC. Problem was, hanging a canopy 40 feet from its support isn’t an easy engineering challenge, as the engineers from CST Covers pointed out. Vince and Allison walk us through the solution.

Allison: To create this century¦s leading designs, architects rely on structural and mechanical engineers to execute their vision. I¦m Allison Toepperwein.

Vince: CST Covers, a company noted for their ability to create domed structures, was hired to engineer and manufacture the canopy for the Francis A. Gregory Library in Washington D.C. The library design featured a huge steel and glass canopy that is cantilevered 40 feet from it¦s support.

Allison: Not only is it cantilevered, but the canopy also features a series of subtle curves, adding extra challenges for the engineering team in both fitting the assembly together and in performing stress calculations.

Vince: You might be surprised to learn that over a span of 40 feet, even rigid steel beams will sag. So in order for the canopy to lie flat in place, it actually has to be engineered with a camber that will then "sag" [finger quotes] flat.

Allison: We spoke with Michael Gamber at CST Covers about what design challenges they faced in engineering a pre-cambered canopy that slopes in multiple directions.

Vince: The architects initially conceived the canopy as a solid welded form, but the design team at CST Covers quickly realized that approach would require materials and manufacturing costs that would crater the budget and timeline.

Allison: So, CST engineers switched to an assembly approach . They developed a "hub and tube" structure that could be assembled on site in stages.

Vince: Because the canopy design curves in multiple directions, creating the pre-cambered effect [steeple fingers and then relax them] for the canopy wasn¦t just a simple task of adjusting the structures connection points along a vertical and horizontal axis.

Allison: Each hub in the canopy had to be designed to fit the pre-cambered shape of the structure. Designing each hub individually would have taken longer than it takes a "Twinky" to grow mold.

Vince: To make sure the canopy was built before the release of "The Fast and the Furious 18", Michael and his team created a "smart model" of the hub that compensated for the slope of the structure along all of its axes¦.

Allison: To have the hub compensate for the canopy¦s slope, a virtual "twisting" [finger quotes] method was developed to find the placement of each subsequent hub and the tubes that would connect the segments.

Vince: CST realized that to achieve the pre-camber the beams would technically "twist" and then straighten back out in their final resting place.

Allison: Because the models of the tubes cannot twist, the twist in the structure was applied to the hub "smart model".

Vince: This extremely complex model mimics the theoretical twist of each tube along its slope, as well as the "sagged" version so that the bolt pattern of each tube and hub connection would be uniform.

Allison: Without this breakthrough, the pre-camber of the canopy combined with its complex geometry would have made connecting each segment too complex to manufacture.

Vince: In the end around 450 hubs, 900 tubes, and 30,000 bolts were needed to ensure that the canopy¦s shape was true to the architects vision.

Allison: Michael Gamber, and his team at CST Covers were able to simplify this process by using PTC¦s Creo software to create their "smart hubs" and tubes.